Three different lines of BALB/c mice are being studied in connection with their incidence of spontaneous mammary tumors (SMT), their expression of mouse mammary tumor virus (MMTV) particles and/or antigens, and their immune response. These lines are: 1) BALB/cCrgl which has low incidence of SMT, is devoid of intact MMTV but expresses some viral antigens and an in vitro immune respone to these antigens as well as to tumor antigens of their syngeneic chemically induced mammary tumors; 2) BALB/cfC3H with a high incidence of SMT and in which MMTV particles can be detected and 3) hereditarily asplenic BALB/c mice heterozygous for the nude gene, which have a high incidence of SMT (45-50%) with a mean age of appearance of 10 months. The proposed research intends to identify, characterize and isolate the immunologically relevent cells known to respond to either viral or tumor antigens and to use them in adoptive transfer experiments. Studies will include the nature of cellular and humoral in vitro reactions and the possible role of suppressor cells. This immunotherapeutic approach aims at achieving protection of mice with a high risk of developing SMT thus getting insight into the mechanism(s) operative in tumorigenesis.